1. Field of the Invention
The present invention relates to a high energy implanter, and more particularly, to an ion source chamber of a high energy implanter with a filtering device.
2. Description of the Prior Art
Doping is a critical step in a semiconductor process. It can be performed by using either diffusion or ion implantation. Ion implantation has proven to be the superior technique as it can produce an improved dopant profile. Also with ion implantation, it is possible to adjust dopant concentration. For these reasons, ion implantation has become the technique of choice for doping in the semiconductor process.
Ions are generated by an ion source chamber of a high energy implanter. These ions are then implanted into a semiconductor wafer. Operations of the ion source chamber are performed by having electrons bombard gaseous molecules causing them to ionize. The ionized molecules are then accelerated through an electric field to strike the semiconductor wafer where they become implanted. This process is carried out at a low pressure as this is necessary to maintain electric current.
Please refer to FIG. 1. FIG. 1 is a perspective view of a prior art ion source chamber 10. The ion source chamber 10 comprises a main chamber 12 for generating ions for
ion implantation, a vent-pipe 13 and a releasing valve 14. The vent-pipe 13 has two open ends: One end is connected to the main chamber 12 and the other end is connected to the releasing valve 14. Air is released through the end of the vent-pipe 13 connected to the main chamber 12. There is also an O-ring tightly clamped between the vent-pipe 13 and the releasing valve 14 for preventing air leakage. The vent-pipe 13 and the releasing valve 14 are necessary for maintaining the pressure in the main chamber 12. The high energy implanter is a vacuum system, therefore, some form of gas must be supplied (e.g. air or nitrogen) to restore the pressure in the main chamber to 1 atm before maintenance. If it is the case, the turbo pump is first stopped, then nitrogen is introduced into the main chamber 12 to increase the pressure. When the pressure exceeds 1 atm, the releasing valve 14 opens and the excess gas is released through the exhaust pipe 16.
During venting of the ion source chamber 10, impurities produced from ions or those shed from the inner wall of the main chamber 12 may become mixed with the nitrogen as it is introduced. Since the releasing valve 14 is located on the underside of the main chamber 12, it is easy for these impurities to drop into the O-ring between the vent-pipe 13 and the releasing valve 14. The impurities thus accumulate and hamper the connection of the vent-pipe 13 with the releasing valve 14. The result is the vent-pipe 13 and releasing valve 14 can no longer form a tight seal with each other not only causing gas leakage to occur but also preventing the high energy implanter from achieving a vacuum. Therefore, periodic cleaning must be performed on the releasing valve 14 which necessitates stopping the turbo pump and removing the releasing valve 14. As it takes a long time to power down or power up the turbo pump, it can be clearly seen that much time and money is wasted cleaning the releasing valve 14.